As early as in 1940's, it has been reported of a gas-phase phosgenation for preparing isocyanates (Siefken, Annalen 562, 108, 1949). A gas-phase reaction is usually carried out in a tubular reactor. This phosgenation reaction is a fast process, which requires fast mixing rate and meanwhile needs to avoid blockage of the reactor at a high reaction temperature as possible. In the gas phase phosgenation at a high temperature, amines and isocyanates are extremely unstable to stay in a relatively long time at 300-500° C., which may cause decompositions like losing NH2 or NCO groups and the like. Thus, on one hand, it needs to strengthen their mixing and reduce their residence time during the reaction process at a high temperature; on the other hand, it needs to conduct the reaction under negative pressure, especially for feedstocks of high boiling points, so as to make amines gasified at a relatively low temperature; furthermore, the obtained high temperature gas mixture needs to be quenched to about 140° C., so as to minimize the loss of products.
EP 0, 289, 840 discloses a cylindrical reactor without any moving parts inside. The reaction is carried out with reactant streams in a turbulent flow state. The gas phase phosgenation of aliphatic amines is a very fast reaction process determined by the mixing velocity. However, due to the back-mixing of reactants, isocyanates may react with amines to form solid by-products deposited on the surface of the reactor, which obstruct its gas flow channel.
U.S. Pat. No. 4,847,408 adopts a reactor where gaseous reactants are mixed and react under a strong turbulent flow state. The reactor has an inner diameter of 2.5 mm and a length of 17.5 mm. The stream of amines is rapidly ejected into the reactor via a nozzle. And HDI is obtained at 400° C. CN 1, 396, 152 improves the reactor described in U.S. Pat. No. 4,847,408 by converting the cylindrical reactor into a mixer of venturi-like in shape. This design may reduce back-mixing and the contact of the gaseous mixture with the inside wall of the reactor.
U.S. Pat. No. 6,082,891 describes a preparation of H6TDI using a microchannel mixer which shows a good result. However, a disadvantage of the reactor is that polymers produced and deposited at a high temperature may block the channel for its small size, and thereby the operation time has to be shortened.
EP 0, 289, 840 and U.S. Pat. No. 4,847,408 describe a condensation of products by absorbing them directly in a solvent. It needs a large solvent container and a big amount of solvents due to the short heat exchange time. Furthermore, quenching and absorption of the high temperature gas mixture may use a heat exchanger. By-products may deposit on surfaces of the heat exchanger, which impairs the heat transfer and finally leads to a blockage of the heat exchanger after a long term operation.
It can be seen from the above comparison that the phosgenation reaction of amines in the gas phase is a fast reaction process, which requires a higher mixing rate to avoid the production of by-products like ureas so as to avoid blockages. The key to obtain a good reaction result is to use a reactor having high mixing efficiency so as to reduce the production of solid by-products. Furthermore, quenching of a high temperature gas mixture also reduces the production of by-products. Therefore, there is a need to find an apparatus and a process for preparation of isocyanates, which provides rapid efficient mixing of reactants and quenching of the high temperature gas mixture after the reaction.